Online Subnet Calculator | IPv4 & IPv6 | VLSM

CIDR	Hosts	Mask
/24	254	255.255.255.0
/25	126	255.255.255.128
/26	62	255.255.255.192
/27	30	255.255.255.224
/28	14	255.255.255.240
/30	2	255.255.255.252

About Subnet Calculator Pro

Professional-grade network calculator. IPv4 CIDR analysis with address type detection. IPv6 sequential planning and /48/56 split strategies. VLSM automatic allocation. Supernetting / CIDR aggregation. Batch lookup, subnet overlap checking, NAT64 mapping and a What's-My-IP tool. All tabular results exportable as CSV or JSON.

Capabilities

IPv4 CIDR — network, broadcast, wildcard, binary/hex breakdown, RFC range detection, router ACL snippets
IPv6 Planner — sequential prefix generation, expandable preview, CSV/JSON export
IPv6 Split — /48 or /56 to /64 with full export
VLSM — automatic allocation from a base block sorted by size, utilisation bar
Supernet — CIDR aggregation to smallest enclosing prefix
Batch Lookup — calculate up to 500 addresses or CIDR blocks in one pass
Overlap Check — detect conflicts between a planned subnet and existing allocations
My IP — see your own connection's IPv4/IPv6 address, never logged
NAT64 Map — convert an IPv4 address to its IPv4-embedded IPv6 form (RFC 6052)

Frequently Asked Questions

What's the difference between a network address and a broadcast address?

The network address is the first address in a subnet (all host bits set to 0) and identifies the subnet itself — it can't be assigned to a device. The broadcast address is the last address in the subnet (all host bits set to 1) and is used to reach every host on that subnet at once. Everything in between is usable for hosts.

What does CIDR notation like /24 actually mean?

The number after the slash is how many bits, counting from the left, are fixed as the "network" portion of the address. A /24 fixes the first 24 bits, leaving 8 bits (256 addresses, 254 usable) for hosts. A smaller number after the slash means a bigger subnet; a larger number means a smaller one.

When should I use VLSM instead of one fixed subnet size?

VLSM (Variable Length Subnet Masking) lets you size each subnet to what it actually needs instead of giving every department the same block. A point-to-point link needing 2 addresses shouldn't consume a /24 meant for 250 hosts — VLSM allocates the largest requirements first and packs smaller ones into the remaining space, so you waste far fewer addresses.

Why would I aggregate (supernet) multiple subnets?

If you have several contiguous subnets that are all routed the same way, advertising them individually bloats routing tables. Supernetting finds the smallest single CIDR block that covers all of them, so one route can replace several — as long as the aggregate doesn't accidentally include addresses that should route elsewhere.

Is any of this sent to or stored on your server?

The addresses you calculate are processed to generate your result and are not logged or archived — see the Privacy Policy for details. Recent-calculation history and your theme preference live only in your own browser's local storage and never reach the server.